The invention relates to a heat exchange element intended for use within a central heating boiler, the heat exchange element being designed as a monocasting from substantially aluminum, the heat exchange element being provided with walls which bound a water-carrying channel, and with at least one wall which bounds at least one flue gas draft to which a burner can be connected, at least one wall which bounds the at least one flue gas draft being water-cooled in that it also forms a boundary for the water-carrying channel, while one said at least one water-cooled wall is provided with heat exchanging surface enlarging pins and/or fins which extend in the respective flue gas draft.
Such a heat exchange element is disclosed in European patent application EP-A-0 889 292. The heat exchange element described therein is particularly intended for great output and is thereto provided with several flue gas drafts. However, prior thereto, applicant has marketed central heating boilers with heat exchange elements with a single flue gas draft. These heat exchange elements are known as type indications W21C Eco and W28C Eco.
The known heat exchange elements have a weight of approximately 0.4 kg/kW. For a heat exchange element of approximately 25 kW, which is a customary output for a normal house, the weight is therefore approximately 10 kg. Moreover, the known heat exchange element with such an output has a water-carrying channel with a content of approximately 2.1 liters. This is, inter alia, the result of the fact that with the known heat exchange elements, the burner is completely surrounded by a heat exchanging surface and associated water channel.
Although the known heat exchange element is relatively small for a boiler with such an output, when the boiler is used for heating not only central heating water but also tap water, the efficiency can be improved still further, and a still more rapid heating of the tap water is desired.
To that end, the heat exchange element of the type described in the opening paragraph is characterized in that the cross-sectional surface of a pin and/or fin mentioned is smaller than 25 mm2.
Previously, it was generally accepted that it was not possible to utilize pins and/or fins with such a small cross-sectional surface, because the pins and/or fins had to have a length of at least 25 mm. This requirement arises from a need for the flue gas draft to have a particular width for discharging sufficient flue gas and because this width is to be completely filled with the heat exchanging surface enlarging pins and/or fins. Pins with such a length need a large cross-sectional surface in view of casting technique requirements. The known pins, for instance, have a length of 25 mm and a diameter of 8 mm. When, with this length, a smaller diameter is chosen, the liquid aluminum solidifies during the casting process before the pin-forming cavity is completely filled. In casting practice, this phenomenon is known as cold flow.
It is noted that from U.S. Pat. No. 5,829,514, a so-called heat sink is known, which is provided with pins with a diameter leading to a cross-sectional surface in a range of the presently disclosed heat exchange element. The known pins have a diameter of 2 mm and, hence, a cross-sectional surface of 3.1 mm2. A heat sink is a device utilized in electronic equipment, such as computers, for cooling electronic components accommodated therein. The heat sink shown in U.S. Pat. No. 5,829,514 comprises a first base plate and a number of pins extending away from this base plate, and a second base plate and a number of pins extending away from this second base plate. After having been manufactured separately in a casting process, the two plates are interconnected. The known heat sink is therefore not a monocasting. Based upon the diameters of the pins mentioned in the text, and the drawings, which are enlarged by a factor of three, the dimensions of the base plates are approximately 3.3*2.5 cm. The separate base plates with pins are releasing, and can therefore be manufactured through die-casting.
With die-casting, the mold is of metal and can be heated, so that the so-called cold flow occurs much less rapidly. Furthermore, the base plate is relatively thick so that virtually no cooling of casting material occurs. Optionally, with such a process, the liquid metal can be supplied under excess pressure. This is contrary to a heat exchange element for a central heating boiler. Firstly, the heat exchanger designed as monocasting is non-releasing. As a result, a mold and cores manufactured from sand, and which are lost after the casting process, have to be utilized. This excludes the possibility of casting under excess pressure. Furthermore, heating a sand cast mold is not possible. When casting a heat exchange element designed as monocasting, the liquid metal runs from one filling point, through the cavities for forming the thin-walled water channels, and thereafter flows into the cavities for forming the pins. Not only is the distance the liquid metal has to travel from the filling point to the pins much longer, because the walls bounding the water channels are thin the extent of cooling of the liquid metal is considerable—which has an adverse affect on the cold flow of the liquid metal in the pin forming cavities. The dimensions of exemplary embodiments of the present invention are in the range of, for instance, 20-50 cm, which is not comparable to the dimensions of the known heat sink.
Also, the freedom of choice of metal used for casting a central heating heat exchange element more limited than the freedom of choice for a heat sink. The central heating heat exchange element is used in an environment of flue gases and water vapor which leads to the formation of highly corrosive acids. With a heat sink air flow is involved. For a heat sink, high-flowing alloys can be selected that can be cast in a simple manner. Such alloys, due to their corrosion sensitivity, are not suitable for the manufacture of a central heating heat exchange element.
For the above reasons, a person of average skill in the field of central heating heat exchanges element design would not turn to the field of heat sinks for guidance in designing boiler heat exchange elements, and in any event such teachings would not suggest the present invention.